1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus that can use the zoom lens. More specifically, the present invention relates to a zoom lens useful in an image pickup apparatus, such as a video camera, a digital still camera, a television (TV) camera, or a monitoring camera, and to an image pickup apparatus including the zoom lens.
2. Description of the Related Art
A four-unit zoom lens, which is constituted by, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power has been known. U.S. Pat. No. 7,830,613 discloses a four-unit zoom lens, whose total size is small, wherein a first lens unit is constituted by a negative lens and a positive lens, and a second lens unit is constituted by negative lens, a negative lens and a positive lens.
Although, U.S. Pat. No. 7,830,613 discloses a zoom lens having a focal length of 28 millimeters (mm), which is the 35 mm equivalent focal length, and a zoom ratio of approximately 6.7, a moving amount the second lens unit is so large that the length of a retractable lens in a retracted state increases. U.S. Pat. No. 7,830,613 also discloses a zoom lens having a focal length of 28 mm, which is the 35 mm equivalent focal length, and a zoom ratio of approximately 4.8. However, the zoom ratio of the zoom lens is not as high as desirable.
In general, a zoom lens that includes lens units each including a larger number of lens elements has a long optical axis for each of the lens units. Moreover, a zoom lens that includes lens units that move large distances during zooming and focusing has a long total length.
In order to reduce the total length of a zoom lens having a predetermined zoom ratio, the refractive power of each lens unit should be increased to thereby decrease the number of lens elements of each lens unit. However, an increase of the refractive power of the lens unit increases changes in aberrations during zooming. Then, it is difficult to compensate for such aberrations in entire zoom range.
In the above-described four-unit zoom lens, in order to simultaneously achieve a compact optical system with a high zoom ratio and good optical performance, it is desirable to appropriately set configurations of lens units, an imaging magnification of the second lens unit, an imaging magnification of the third lens unit, and a refractive power of the second lens unit.